By far, the most commonly used method for sparse optical flow is LK [1]}.
This method is mathematically well defined and has clear assumptions for proper functioning like no photometric deformations, small displacement (or the images are separated by a very small time increment), and the point is a corner.
Since this method is computationally cheap, it can be applied in many real-time feature tracking applications.
Within the last 5 years applications of LK include image alignment [2]}, image registration [3]}, feature tracking [4]}, [5]}, visual odometry [6]}, and motion breath recognition [7]}.
In 2015, this method was used for facial expression recognition [8]}.
